Fixup Base Slash Literal Lexing

Break the ambiguity with bare slash regex literals that have any
amount of whitespace after the slash delimiter. These now lex as
spaced binary operators as appropriate.

Fixes #627

rdar://98736315

Author: CodaFi

Sources/SwiftParser/Lexer.swift

@@ -2015,6 +2015,20 @@ extension Lexer.Cursor {
       return nil
     }
 
+    // For `/.../` regex literals, we need to ban space and tab at the start of
+    // a regex to avoid ambiguity with operator chains, e.g:
+    //
+    // Builder {
+    //   0
+    //   / 1 /
+    //   2
+    // }
+    //
+    if poundCount == 0 && !Tmp.isAtEndOfFile &&
+        (Tmp.peek() == UInt8(ascii: " ") || Tmp.peek() == UInt8(ascii: "\t")) {
+      return nil
+    }
+
     var isMultiline = false
     while !Tmp.isAtEndOfFile {
       switch Tmp.peek() {

Tests/SwiftParserTest/LexerTests.swift

@@ -474,6 +474,31 @@ public class LexerTests: XCTestCase {
       ])
     }
   }
+
+  func testNotARegex() {
+    var data =
+    """
+    min(reduced.count / 2, chunkSize / 2)
+    """
+    let lexemes = data.withUTF8 { buf in
+      Lexer.lex(buf)
+    }
+    AssertEqualTokens(lexemes, [
+      lexeme(.identifier, "min"),
+      lexeme(.leftParen, "("),
+      lexeme(.identifier, "reduced"),
+      lexeme(.period, "."),
+      lexeme(.identifier, "count ", trailing: 1),
+      lexeme(.spacedBinaryOperator, "/ ", trailing: 1),
+      lexeme(.integerLiteral, "2"),
+      lexeme(.comma, ", ", trailing: 1),
+      lexeme(.identifier, "chunkSize ", trailing: 1),
+      lexeme(.spacedBinaryOperator, "/ ", trailing: 1),
+      lexeme(.integerLiteral, "2"),
+      lexeme(.rightParen, ")"),
+      lexeme(.eof, ""),
+    ])
+  }
 }
 
 extension Lexer {